1. Field of the Invention
The present invention relates to an image processing method of converting input image data of M-gradations in a pixel into multivalued image data of N-gradations (M>N>2) in a pixel by a pseudo half tone processing.
2. Description of the Related Art
In an image forming apparatus such as a printer, copying machine or the like, an image processing of converting multivalued input image data into a binary or multivalued image signal is carried out. As such an image processing, an error diffusion method that has excellent gradation reproducibility and high resolution, for example, is known. A case where a binary processing is performed with use of the error diffusion method will be described, for example. A binarized pixel distributes the result obtained by multiplying a binary error generated in the binary processing by a certain weighting factor to surrounding pixels. Then, a binary processing is performed to an input remarked pixel by adding the entire errors distributed to the remarked pixel and using a threshold.
Note that, when a multivalued processing more than a ternary processing is performed, the processing is carried out with use of a plurality of thresholds that corresponds to the multivalue.
By performing the binary processing or multivalued processing to the input image with use of the error diffusion method, an average density can be theoretically preserved.
However, even in the image processing using such error diffusion method, when overall printing, which is a moderate gradation or a solid color, is performed, dots having the same number of gradations may be formed adjacently to one another.
On the other hand, a dot to be practically printed has a shape different from rectangular one that is theoretically considered. For example, an eight-valued case for an ink jet printer is considered. As shown in FIG. 11, practical dots are reproduced using variable dot sizes of seven kinds for one pixel. In general, the size of each dot in each gradation is adjusted in advance to have a linear characteristic etc. in density. However, when pixels having fifth-gradations are printed successively, it is desirable that edges of the dots are secured as shown with H in FIG. 12. In a practical case, the adjacent dots are associated with each other as shown with I in FIG. 12. With the influence of the association, a dot area increases remarkably. That is, a surplus area d is generated owing to the association, and the density increases rapidly. A phenomenon such that a rapid change in density occurs in a case where the specific numbers of gradation are adjacent to each other is referred to as a tone jump. The phenomenon occurs not only in an ink jet printer but also in the other image forming apparatuses.
Therefore, in the Jpn. Pat. Appln. KOKAI Publication No. 2000-333007 for example, input image data are processed by the error diffusion method. After the processing, by applying correction to reduce the dot area of the remarked pixel with use of the peripheral dot information, the association of dots is prevented from occurring. More specifically, assume that output image shown in FIG. 13A is required. When data after an error diffusion processing are output data themselves as shown in FIG. 13A, an area surplus e occurs as shown in FIG. 13B. It is checked whether a dot exists in eight pixels surrounding the remarked pixel. Then, the correction as shown in FIG. 13C is performed following correction regulations shown in FIG. 14 against the remarked pixel and its surroundings, thereby preventing an influence from the area surplus.
There are two problems in the image processing method disclosed in the Jpn. Pat. Appln. KOKAI Publication No. 2000-333007. One problem is such that the correction is carried out after the error diffusion processing, so the average density cannot be preserved. The other problem is that compatibility between a recording medium and ink, such as bleeding etc., is not considered, so it is not possible to deal with changes of recording medium. The second problem is not a problem in a case using a single recording medium. However, in a practical case of an image forming apparatus that performs printing on recording media of various kinds, a predetermined gradation, in which the tone jump occurs when the identical gradations are adjacent to each other owing to characteristics of recording medium and ink, is changed. The method cannot deal with such a change of the predetermined gradation.
Accordingly, an object of the present invention is to provide an image processing method that enables avoidance of the tone jump owing to the area increase caused by dot association and preservation of the average density after converting into multivalued image data.